As an information recording medium, a non-contact type optical recording medium, having a high recording density, is now in use. The information is recorded on the optical recording medium by illuminating a light beam, such as a laser light beam, on the recording surface, while the information recorded on the optical recording medium may be reproduced in similar manner. As this sort of the optical recording medium, a recording medium formed an optical disc, that is a disc-shaped optical recording medium, is finding widespread use because of ease in retrieving the recorded information.
An optical disc has a spirally or concentrically formed recording track(s). The distance between center lines of neighboring recording track turns, that is the track pitch, is approximately 1.6 μm in the case of for example a CD (Compact Disc), while being smaller and 0.74 μm in the case of a DVD (Digital Versatile Disc), for thereby improving the information recording density appreciably.
For illuminating the laser light on an optical disc where the track pitch on the recording tracks of a DVD, etc. is decreased to improve the information recording density, a beam spot smaller than in the case of an optical disc with a larger track pitch needs to be formed on the recording surface of the optical disc.
The diameter of the beam spot of the laser light beam, converged by the objective lens, is proportionate to the design wavelength of the laser light, while being inversely proportionate to the numerical aperture (NA) of the objective lens. Thus, for reducing the beam spot diameter, it is necessary to increase the numerical aperture of the objective lens and to decrease the laser light wavelength.
For recording the information on an optical disc by a phase change method, or still other methods, the laser light of high light energy is required. On the other hand, the laser light noise due to reflected laser light needs to be reduced. For this reason, the driving power is varied by such methods as by superposing high frequency on the driving current or voltage of the semiconductor laser to vary the wavelength of the laser light in a shorter period. Consequently, in an optical pickup designed for illuminating a coherent laser light beam on the optical disc, chromatic aberration ascribable to wavelength variations of several nm tends to be produced, thus increasing the size of the beam spot on the optical disc.
An optical pickup includes an objective lens for converging a laser light beam 200 on the recording surface of an optical disc. A state-of-the-art objective lens 201, shown in FIG. 1, is formed by a sole lens, obtained on glass molding, and has an aspherical surface 201a of high light converging performance.
As may be seen from the graphs of the spherical aberration of FIG. 2A, astigmatic aberration of FIG. 2B and the distortion aberration of FIG. 2C, the chromatic aberration of the order of ±0.6 μm/nm is generated with this objective lens 201 for the wavelength variation of the order of ±0.2 nm, even with the use of an aspherical surface 201a. It should be noted that, in the aberration diagrams of FIGS. 2A to 2C, solid lines, broken lines and chain-dotted lines indicate the values of the aberration at 405 nm, 403 nm and 407 nm, respectively, and that, in FIG. 2B, showing astigmatic aberration, thick lines and fine lines indicate values in a sagittal image surface and in a tangential image surface, respectively.
For recording the information on an optical disc, in which high recording density is achieved by narrowing the track pitch of the recording medium, it is desirable to converge the laser light to close to the diffraction threshold by an objective lens to form a smaller beam spot. However, with the state-of-the-art monolithic objective lens 201, obtained on molding the glass having the aspherical surface 201a, it is difficult to converge the laser light to close to the diffraction threshold, because of generation of the chromatic aberration, as described above.